1. Field of the Invention
The present invention relates to a method for producing a low dielectric constant (low-k) film, and specifically, relates to a method for producing a low-k film comprising porous silica. The present invention also relates to a semiconductor device including a low-k film produced by the method, and a method for manufacturing the semiconductor device.
2. Description of the Related Art
Highly integrated semiconductor circuits with ultra-high speed have caused a problem of increase in signal delay time and power consumption in a multilevel interconnect structure. The signal delay time is determined by the resistance-capacitance (RC) delay, which is a product of an interconnect resistance and an interconnect capacitance. The interconnect resistance increases when an interconnect width is reduced, and the interconnect capacitance increases when an interconnect pitch is reduced. Therefore, the signal delay time increases along with miniaturization. Moreover, the power consumption is determined by a product of a clock frequency, an interconnect capacitance, and a supply voltage. Lowering of a supply voltage is effective for reduction of the power consumption. However, the generation of LSIs having a minimum working dimension of 90 nm, for example, already has the supply voltage of approximately 1 V. Due to restrictions on threshold voltage at which a MOSFET can operate, further significant reduction in the supply voltage can no longer be expected. Therefore, the interconnect capacitance needs to be reduced unless the operation clock frequency is reduced.
Two methods are effective for reduction of the signal delay time and the power consumption: one is to reduce the interconnect resistance, and the other is to reduce the interconnect capacitance. In order to improve the interconnect resistance, introduction of a cupper interconnect having a resistivity lower than a resistivity of an aluminum interconnect has been advanced. On the other hand, in order to improve the interconnect capacitance, a porous silica (porous SiO2) film as a low-k material has been investigated as an alternative to bulk SiO2, a material that has conventionally been used as an interlayer insulating film in an interconnect.
It has been thought that porosification is an essential technique in order to attain a material having a lower dielectric constant, and various methods for forming a porous film have been proposed. JP-A-2005-210111 has proposed a method for improving mechanical strength of a low-k film made of a porous film, in which a material for forming a low-k film comprising a first particulate which contains a combination of a silicon atom and an oxygen atom around an organic polymer and to which second particles having a pore are adhered. While the mechanical strength improves, the dielectric constant depends on the resin of a bulk. The dielectric constant of the formed insulating film itself has not been evaluated at all, either. JP-A-2007-149954 has disclosed a method in which a siloxane oligomer is deposited on a substrate by a plasma polymerization reaction using a plasma CVD apparatus, and an organic amine (tetrapropylammonium aqueous solution) is made to act on the deposited siloxane oligomer to form a porous film. JP-A-2004-311532 has disclosed a method for forming a porous film by heat treating a precursor formed on substrate by a coating method multiple times.
However, formation of the porous film causes such problems as reduction in mechanical strength as a solid body and increase in adsorbed water.
As an example of formation of the porous film, when a porous silica film is used as the porous film, a silica precursor material (TEOS, etc.) containing a pore generator is used to form a thin film on a substrate. Subsequently, the substrate is heat treated so that the pore generator volatilizes and pores are formed in the thin film. However, hydrophilic Si—OH bonds are formed on the pore surface after heat treatment, and water absorption that takes place for a very short time in the process causes increase in the dielectric constant and deterioration of leakage current. Additionally, there is a problem of difficulties in control of a pore diameter so that mechanical strength reduces.